DUBAI/RIYADH (Reuters) - Differences
between the United States and Saudi Arabia over Middle East policy
persist, despite attempts to shore up their old alliance, and may prove
calamitous for Syrian rebels.

Although there is evidence that some American weapons are starting
to find their way to more moderate groups fighting Syrian President
Bashar al-Assad, disagreements over what to supply, and to whom,
have hindered the fight.

Rebels lament a lack of anti-aircraft missiles to help counter
Assad's air force.

Saudi Arabia and Qatar have been funding the rebels for years now,
arguing that the war in Syria is a battle for the future of the
Middle East, pitting pro-Western forces against Riyadh's main enemy
Iran and Islamist militants.

However, while the administration of U.S. President Barack Obama
also blames Assad for the violence and wants him to leave power, it
sees the conflict very differently.

American officials fear involvement in a messy civil war for which
they see no clear military solution and which threatens to
radicalize a new generation of Islamists who hate the West.

Among the rebels, the failure of the Saudis and the Americans to
cooperate better stirs disillusion. Two hours of talks between Obama
and Saudi King Abdullah in March appear to have done little to alter
that sentiment.

In private, American and Saudi officials defend a relationship that
in many ways remains strong and broad-based.

But they acknowledge a fundamental divergence over how to approach
big political conflicts in the Middle East that were aggravated by
the Arab spring, particularly what Riyadh sees as Iranian
expansionism across the region.

When Washington agreed a preliminary deal with Tehran in November
over its nuclear program, Riyadh feared it would reduce political
pressure on Iran, giving it more scope to push its interests across
the region.

The Saudis were also angry when Obama did not do more to back
Egypt's Hosni Mubarak who was forced from power in 2011, and when
Washington criticized the army for ousting his successor, Mohamed
Mursi of the Muslim Brotherhood.

Meanwhile Assad appears to be gaining ground and has told a Russian
official the heavy fighting will be over within a year.

"I'm afraid that what remains of the Syrian state will vanish, so I
would say the United States has had a big failure in this regard,"
said Abdullah al-Askar, chairman of the foreign affairs committee
for Saudi Arabia's appointed Shoura Council, which advises the
government.

But if the Saudis felt stymied, so did the Americans.

"The frustration with the Saudis was that they never gave us a
plan," a former senior U.S. diplomat who worked in the region told
Reuters.

The former diplomat said there had to be a strategy that included
pulling the opposition together into a political and military union
dominated by moderates, while arm-twisting Assad's main backers in
the Security Council: Russia and China.

DISILLUSIONMENT

"There's got to be something more than throwing weapons and
suitcases of money," the former diplomat said.

Riyadh's main Syria strategy has been based on persuading Washington
of the need to bring its far greater diplomatic, military and
planning clout to bear in helping the rebellion.

"We want the Americans to use their Tomahawks and F16s and beat the
hell out of Bashar al-Assad. But at the same time I can see the
Americans saying to the Saudis 'You guys have F15s too'," said Jamal
Khashoggi, head of a Saudi television news channel owned by a nephew
of King Abdullah.

The United States fears that any heavy weapons or training for the
rebels might leak to militants who would then turn on the West,
repeating the U.S. experience in 1980s Afghanistan.

While Riyadh
is aware of the danger of militant blowback - as happened a decade
ago with an al Qaeda campaign of attacks in the kingdom - it sees
U.S. reluctance as a strategic error.

Saudi officials think the failure to back moderate rebel groups
earlier not only encouraged Assad, but allowed militants to emerge
as the strongest element in the opposition.

Although Saudi authorities repeatedly announced that donations to
Syrian rebel or humanitarian groups should only go via official
channels to ensure they did not end up in militant hands, some
private donations were likely made to radicals.

Officials in the kingdom were frustrated at what they saw as
American dithering, particularly after Obama backed down from a
strike on Syria following a chemical weapons attack in the Damascus
suburbs in August.

INTELLIGENCE CHIEF

Still, cooperation has improved in recent months, American officials
believe, and U.S.-made rockets have started to appear on the
battlefield.

One reason for the better atmosphere between the allies may be the
departure from office this year of intelligence chief, Prince Bandar
bin Sultan, who was running Saudi policy on Syria.

His abrasive manner and sometimes erratic way of working caused
friction with the Americans.

However, Washington still wants more openness on the Saudi side,
said a diplomatic source in the Gulf, one of several interviewed for
this article.

But the source added that the Saudis still felt left out on a limb
by last year's non-strike.

"They see brush fires all around them and are concerned Washington
is not doing more to help the Syrian opposition," the diplomatic
source said.

For all its differences with Riyadh, Washington remains the
kingdom's most important ally, sharing an outlook that values
regional stability and a tough approach to Islamist militants.

A big American military presence in the Gulf still protects Saudi
borders from foreign enemies, the kingdom's armed forces are
equipped mostly with American hardware and a web of personal
relations binds officials, diplomats and businessmen.

But after the Arab Spring destabilized one of Saudi Arabia's
neighbors after another, Riyadh's perception of a pivotal threat was
not matched in Washington.

ROOT OF UNEASE

The former U.S. diplomat said this still influenced Riyadh's view of
Washington's nuclear talks with Iran, despite attempts by Obama and
other officials to assuage Saudi fears.

Gary Grappo, a former deputy chief of mission in Riyadh, said the
Saudis were intensely suspicious of Iran.

"There was an overwhelming obsession with Iran and the threat that
it posed. We heard from Saudi officials, some quite senior, that
Iran's intention is to position itself as leader of the Muslim
world, especially after the Shi'ites re-established control over
their holy sites in Iraq - Kerbala and Najaf."

"It sounds like an exaggeration to us, but I heard it: 'The next
destination is Mecca and Medina'," said Grappo.

Though frayed, the alliance is unlikely to break, with the former
diplomat describing U.S.-Saudi relations in terms of a longstanding
marriage:

"It's like a couple that's been married for 40 years - you can't
imagine not being together, but you can't seem to avoid poking each
other."

(Additional reporting by Mariam Karouny in Beirut; Editing by
William Maclean and Giles Elgood)